Method for making Mo4 S4 L6 (C-2387)

ABSTRACT

The present invention provides an improved method for preparing compounds of the formula Mo 4  S 4  L 6  comprising: 
     contacting a compound having the formula Mo 2  S 4  L 2 , wherein L is a 1,1-dithioacid ligand, with a reducing agent having a reduction potential sufficient to reduce Mo(V) to lower oxidation states, especially to Mo(III) and Mo(IV), at a temperature and for a time sufficient to form the Mo 4  S 4  L 6  compound. Preferably, the Mo 2  S 4  L 2  compound is dissolved in an organic solvent along with the reducing agent and the solution is heated at temperatures above 25° C., up to the boiling point of the solvent and, more preferably, at temperatures in the range of from about 50° C. to about 250° C.

FIELD OF THE INVENTION

This invention relates to improvements in the synthesis of Mo₄ S₄ L₆ compounds.

BACKGROUND OF THE INVENTION

Molybdenum compounds having a thiocubane structure are produced by a variety of methods. For example, T. Shibahara et al, J. Am. Chem. Soc., Vol. 106, pp. 789-791 (1984) discusses a method for making the [Mo₄ S₄ (edta)₂ ]3--ion containing species by reacting a water soluble Mo(V) dimer in HCl. P. Kathirgamanathan et al, J. Chem. Soc., Chem. Commun., pp. 953-954 (1985), describes electrochemically reducing a Na₂ [Mo(V)₂ S₂ O₂ (cysteine)₂ ]·3H₂ O in HCl to form (Me₄ N)₅ [Mo₃ S₄ (NCS)₉ ] and the tetramer (Me₄ N)₇ [Mo₄ S₄ (NCS)₁₂ ]. P. Kathirgamanathan et al, J. Chem. Soc., Chem. Commun., pp. 1437-1439 (1985), describes preparing mixtures of (Me₄ N)₅ [Mo₃ X₄ (NCS)₉ ] and (Me₄ N)₇ [Mo₄ X₄ (NCS)₁₂ ] compounds, where X is sulfur or oxygen. More recently, in U.S. Pat. No. 4,990,271 there is described a method for making thiocubane Mo compounds having the formula Mo₄ S₄ (ROCS₂)₆ by reacting molybdenum hexacarbonyl, Mo(CO)₆, with a xanthogen disulfide.

Notwithstanding the plethora of methods for preparing molybdenum containing thiocubane type compounds, there remains a need for a preparative method that is more simple and less expensive.

It is, therefore, an object of the present invention to provide an improved method for forming thiocubane Mo compounds of the general formula Mo₄ S₄ L₆, where L is a dithioacid ligand.

SUMMARY OF THE INVENTION

Accordingly, there is provided an improved method for preparing compounds of the formula Mo₄ S₄ L₆ comprising:

contacting a compound having the formula Mo₂ S₄ L₂, wherein L is a 1,1-dithioacid ligand with a reducing agent having a reduction potential sufficient to reduce Mo(V) to lower oxidation states, especially to Mo(III) and Mo(IV), at a temperature and for a time sufficient to form the Mo₄ S₄ L₆ compound. Preferably, the Mo₂ S₄ L₂ compound is dissolved in an organic solvent along with the reducing agent and the solution is heated at temperatures above 25° C., up to the boiling point of the solvent and, more preferably, at temperatures in the range of from about 50° C. to about 250° C.

DETAILED DESCRIPTION

In a preferred method of the present invention, a compound having the formula Mo₂ S₄ L₂, wherein L is a 1,1-dithioacid ligand, is added to a sufficient amount of an organic solvent to form a solution.

In general, any dithioacid ligand may be used. Thus, L may be a dithiocarbamate, xanthate, thioxanthate, dithiophosphate, dithiophosphinate, or other similar dithioacids and mixtures thereof. Preferably, the ligands, L, will have organo groups having from about 1 to 30 carbon atoms. For example, when L is a dithiocarbamate, (S₂ CNR₂ -), or a xanthate, (S₂ COR-) the organo group R preferably will have from 1 to 30 carbon atoms.

The Mo₂ S₄ L₂ compound can be prepared by generally known techniques.

Any organic solvent capable of dissolving the Mo₂ S₄ L₂ compound may be used in the method of this invention. Preferably, the organic solvent chosen will also be capable of dissolving the reducing agent used in the process. In general, hydrocarbons, ethers and formamides are useful. Especially useful are organic solvents that have boiling points above about 50° and in the range, for example, of from about 50° C. to about 250° C. Indeed, aromatic hydrocarbons, such as toluene or xylene, or other solvents, such as tetrahydrofuran, dimethylformamide and mixtures thereof are most preferred organic solvents for use in this invention.

Thus, a solution of the Mo₂ S₄ L₂ compound and the reducing agent is prepared. The reducing agent may be any compound with a reduction potential sufficient to reduce the Mo(V) in the Mo₂ S₄ L₂ compound to a lower oxidation state, such as Mo(III) and Mo(IV). Suitable reducing agents include Zn, Mg, NaBH₄, dithionite salts, LiR₃ BH, R₄ NBH₄, where R is an alkyl group containing 1 to about 30 carbon atoms, and mixtures thereof.

The mole ratio of reducing agent to Mo₂ S₄ L₂ compound employed will range generally from about 0 10 to about 100 and, preferably, from about 0.25 to about 10.

Optionally but preferably, a disulfide of a 1,1-dithioacid is added to the solution of the Mo₂ S₄ L₂ compound and reducing agent. In general, from about 0.1 to about 10 moles of disulfide per mole of dimer is added and, preferably, about 2 moles of disulfide per mole of Mo₂ S₄ L₂ compound. The additional disulfide improves the yield obtained in converting the Mo₂ S₄ L₂ compound to the Mo₄ S₄ L₆ compound. Also, the disulfide of the dithioacid added to the solution preferably is a disulfide of xanthates, dithiocarbamates, dithiophosphates, dithiophosphinates and the like.

In yet another embodiment of the invention, a salt of any of the 1,1-dithioacids, L, previously mentioned can be added to the solution of the Mo₂ S₄ L₂ compound and reducing agent. Typical salts include alkali metal, alkaline earth metal, ammonium and alkylammonium salts. In general, these will be added in amounts ranging from about 0.1 to 10 moles of salt per mole of Mo₂ S₄ L₂ compound and, preferably, about 0.5 to about 2 moles per mole of Mo₂ S₄ L₂ compound.

After forming the solution, as outlined above, in some instances a reaction may occur at ambient temperature and heating of the reactants will be unnecessary. In general, however, the solution is heated at a temperature and for a time sufficient to form the Mo₄ S₄ L₆ compound. Typically, the solution will be heated at a temperature above room temperature up to the boiling point of the solvent. More typically, the solution will be heated at temperatures in the range of from about 50° C. to about 250° C. The time of heating will depend upon a number of factors, such as the reducing agent, the solvent and the temperature employed. In general, however, the solution will be heated for times ranging between about 0.5 hours to about 24 hours or more.

The Mo₄ S₄ L₆ product can be isolated readily from the heated solution by any number of well known techniques. In some instances, the product will precipitate from solution and, hence, can be separated and recovered by filtration. In other instances, the solvent can be removed, for example, in vacuo, and the crude product will remain. The crude product can, of course, be purified by recrystallization, column chromatography or the like.

The following examples will serve to illustrate specific procedures used in accordance with the claimed invention.

EXAMPLE 1 Synthesis of Mo₄ S₄ [(C₄ H₉)₂ NCS₂ ]₆ by Reduction with NaBH₄

Mo₂ S₄ [((C₄ H₉)₂ NCS₂ ]₂ (50 mg, 0.7 mmol) and tetraisobutylthiuramdisulfide (14 mg, 0.35 mmol) were dissolved in dimethylformamide (4 ml), forming a solution. The solution was combined with a sodium borohydride (10 mg, 0.094 mmol) solution in 4 ml DMF and heated to 120° C. for 1 hour. A purple colored Mo₄ S₄ [(C₄ H₉)₂ NCS₂ ]₆ was recovered in 35% yield by silica column chromatography using a solution of 75% CH₂ Cl₂ :25% hexane as the eluent.

EXAMPLE 2 Synthesis of Mo₄ S₄ [(C₄ H₉)₂ NCS₂ ]₆ by Reduction with LiEt₃ BH

Mo₂ S₄ [(C₄ H₉)₂ NCS₂ ]₂ (100 mg, 0.14 mmol)was dissolved in tetrahydrofuran (7 ml) and degassed. Next, 0.15 ml of a 1 M solution of LiBEt₃ H in tetrahydrofuran (0.15 mmol) was added with a syringe to the Mo₂ S₄ [(C₄ H₉)₂ NCS₂ ]₂ solution. The resulting solution was stirred for 30 minutes and Mo₄ S₄ [(C₄ H₉)₂ NCS₂ ]₆ was recovered from solution in 12%-15% yield by silica column chromatography using a solution of 75 vol. % CH₂ Cl₂ and 25 vol. % hexane as the eluent.

EXAMPLE 3 Synthesis of Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ by Reduction with NaBH₄

Mo₂ S₄ [(C₈ H₁₇)₂ NCS₂ ]₂ (50 mg, 0.052 mmol) and sodium borohydride (8 mg, 0.075 mmol) were dissolved in dimethylformamide (4 ml) and heated to 120° C. for hour. The Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ compound (purple in color) was identified in the solution by thin layer chromatography.

EXAMPLE 4 Synthesis of Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ by Reduction with NaBH₄ in the Presence of Na[(C₈ H₁₇)₂ NCS₂ ]

Mo₂ S₄ [(C₈ H₁₇)₂ NCS₂ ]₂ (50 mg, 0.052 mmol) and the sodium salt of dioctyldithiocarbamic acid (8 mg, 0.024 mmol) were dissolved in dimethylformamide (3 ml) and added to the sodium borohydride (8 mg, 0.075 mmol) in 2 ml of dimethylformamide. The mixture was heated to 120° C. for 1 hour. A purple Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ formed and was identified in the solution by thin layer chromatography.

EXAMPLE 5 Synthesis of Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ by Reduction with NaBH₄ in the Presence of Tetraoctylthiuram Disulfide

Mo₂ S₄ [(C₈ H₁₇)₂ NCS₂ ]₂ (50 mg, 0.052 mmol), tetraoctylthiuram disulfide (17 mg, 0.027 mmol) and sodium borohydride (8 mg, 0.024 mmol) were heated together at 100° C. in dimethylformamide (5 ml) for 1 hour to form a solution. A 40% yield of purple Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ was recovered from the solution by silica column chromatography using a CH₂ Cl₂ eluent.

EXAMPLE 6 Synthesis of Mo₄ S₄ [(C₃ H₇)₂ NCS₂ ]₆ by Reduction with [(C₄ H₉)₄ NBH₄ ] in the Presence of Tetrapropyl Thiuram Disulfide

Mo₂ S₄ [(C₃ H₇)₂ NCS₂ ]₂ (100 mg, 0.15 mmol), tetrapropyl thiuram disulfide (105 mg, 0.30 mmol) and tetrabutylammonium borohydride (191 mg, 0.59 mmol) were heated together at 115° C. in toluene (20 ml) for 4 hours. A 10% yield of purple Mo₄ S₄ [(C₈ H₁₇)₂ NCS₂ ]₆ was recovered from the solution by silica column chromatography using CH₂ Cl₂ as the eluent. 

What is claimed is:
 1. A method for making a compound having the formula Mo₄ S₄ L₆ comprising:contacting a compound having the formula Mo₂ S₄ L₂, wherein L is a 1,1-dithioacid ligand, with a reducing agent having a reduction potential sufficient to reduce Mo(V) to lower oxidation states, the contacting being conducted at a temperature and for a time sufficient to form the Mo₄ S₄ L₆ compound.
 2. The method of claim 1 wherein the contacting is conducted in a solution.
 3. The method of claim 2 wherein the ratio of reducing agent to Mo₂ S₄ L₂ compound is in the range of from about 0.10:1 to about 100:1.
 4. The method of claim 3 wherein the reducing agent is selected from the group consisting of Zn, Mg, NaBH₄, LiR₃ BH, R₄ NBH₄ where R is an alkyl group of from 1 to about 30 carbon atoms, dithionite salts and mixtures thereof.
 5. The method of claim 4 wherein the solution is heated at a temperature above about 25° C. to about 250° C.
 6. The method of claim 5 wherein the reducing agent is NaBH₄.
 7. The method of claim 5 wherein the reducing agent is LiR₃ BH.
 8. The method of claim 5 wherein the reducing agent is R₄ NBH₄.
 9. The method of claim 5 including adding a disulfide of a 1,1-dithioacid to the solution in amounts ranging from about 0.1 to about 10 moles of disulfide per mole of Mo₂ S₄ L₂.
 10. The method of claim 5 including adding a salt of a 1,1-dithioacid to the solution in amounts ranging from about 0.5 to about 2 moles of salt per mole of Mo₂ S₄ L₂ compound.
 11. The method of claim 9 wherein the ligand, L, of the Mo₂ S₄ L₂ compound and the disulfide of the 1,1-dithioacid are selected from the group consisting of dithiocarbamates, xanthates, thioxanthates, dithiophosphates, dithiophosphinates or mixtures thereof.
 12. The method of claim 10 wherein the salt of the 1,1-dithioacid is selected from alkali, alkaline earth, ammonium, alkylammonium salts and mixtures thereof.
 13. The method of claim 11 wherein the ligand and the 1,1-dithioacid are dithiocarbamates. 